An aqueous solution containing a surfactant known as Aqueous Film Forming Foam (AFFF) is often required when fighting fires of flammable liquid fuel. That foam forms an air-excluding vapour-suppressing aqueous film on the surface of the fuel. This foam is generated by pumping a concentrated solution of AFFF from reservoir tanks through a foam/liquid proportioner where it is mixed with water, either fresh or seawater, in a particular ratio. It is important that correct proportioning is maintained in making a foam that has a proper expansion to form a fire-resistant foam. If proper proportioning is not maintained, then the resulting foam may be ineffective in fighting a fire or more AFFF concentrate than actually required may be used. If more concentrate than required is used, then this would shorten the time before the amount of AFFF concentrate in the reservoirs is exhausted which reduces the effective fire fighting time.
Several methods are currently used to determine the concentrating of AFFF from the foam/liquid proportioner discharge. One method is by using a conductivity meter. However, differences in temperature may change conductivity of the solution. Thus, careful calibration and temperature compensation procedures are required for most of these meters. Furthermore, conductivity meters cannot be used when seawater is used for generating the fire fighting foam due to the conductivity that is already present in sea water.
On navy ships, the capability to effectively fight on-board fires caused by flammable liquids such as aviation turbine fuel, naval distillate fuel, lubricating oils and hydraulic fluids is provided through the use of AFFF concentrate. That AFFF concentrate is pumped, from reservoirs for the concentrate, through a foam/liquid proportioner where it is mixed with sea water in a predetermined ratio by injecting a metered amount of concentrate into the discharge stream from the foam/liquid pump. To generate an effective fire fighting foam, it is essential that the proper ratio of concentrate to sea water is maintained in the discharge from the pump.
A 6% foam solution is considered to be an optimum concentration for one type of AFFF to provide effective control of a liquid hydrocarbon fire. The resulting foam markedly increase the effective volume of the water and, by reducing the surface tension, the surfactant increases the ability of the water to wet other surfaces instead of beading. If the concentration of AFFF in the pump discharge drops below 6%, the ability of the foam to smother a fire is greatly reduced while the concentrate is used at an accelerated rate when the ratio raises above 6%. That accelerated rate will shorten the length of time before the supply of concentrate in their reservoirs is exhausted. This could create a serious problem on ships where a new supply of AFFF may not be readily available for a lengthy period of time. Therefore, it is essential to be able to determine the concentration of the liquid solution at a pump's discharge in order to provide proper adjustment of the foam/liquid proportioner and obtain an optimum fire fighting concentration in the discharge.
The use of a conductivity meter to determine the % concentration of AFFF at a pump's discharge is, as previously mentioned, not suitable when seawater is being used to generate the fire-fighting foam. Furthermore, naval proportioning systems are vulnerable to corrosion by seawater and are, as a result, the ones most in need of regular overhaul and monitoring. This frequently results in dockyard laboratories being requested to determine the concentration in the discharge from proportioning pumps when new pumps are installed or when pumps are overhauled or are being adjusted. One method that laboratories may use to determine the concentration is with a refractive index (RI) detector. This entails preparing varying standard sample solutions, ones containing 1, 2, 3, 4, 5, 6 and 7% AFFF by volume concentrations, with their refractive indexes being determined using a flow through refractive index detector. The refractive index for the samples, measured in millivolts (MV) at the detector's output, are recorded by a strip chart recorder to provide a visual representation of the instruments response to the samples. Those visual representations are then plotted against the solution concentration of the standard solutions in order to obtain calibration curves for the instrument. The response of the instrument to a sample from a pump's discharge is then measured and that response compared with the previously prepared calibration curve. The % concentration at the output from the pump can be determined from where the measured output of the RI detector to that sample fits on the calibration curve. This is a time consuming and expensive procedure since it can only be carried out with the required accuracy in a laboratory using large non-portable instruments. This poses a logistical problem for ships, in particular, as well as for many fire departments and air bases since it can take several days to obtain results from the laboratory as to the percentage of AFFF in the foam being generated at the output of the pump.
A very low refractive index is exhibited by these liquid solutions with only a very small change in the refractive index occurring with a change in concentration of AFFF solutions (0.005 RI units over a 0% to 9% range). Bench top refractive index detectors, such a described above, are capable of detecting a 1 part in 10.sup.7 change in refractive index which is suitable for determining the % concentration in a sample solution at the required resolution. Handheld manual prism refractometers have been used to measure the refractive index and determine the % concentration. These hand-held refractometers also require the user to plot the refractive indices of prepared standards and unknowns versus concentration to determine the AFFF concentration which is a time consuming process. Tests have determined that these refractometers can, at best, measure changes in refractive index in the order of 1 part in 10.sup.3 (versus 1 part in 10.sup.7 for a benchtop refractor) and this is insufficient sensitivity to detect the very small changes in refractive index necessary to determine the % concentration of AFFF solutions at the pump's discharge with the required accuracy.